The present invention relates to an objective lens for an optical pick-up of an optical disc drive, and more particularly to an objective lens having an NA (numerical aperture) of 0.7 or more. The invention also relates to an optical pick-up employing such an objective lens.
The NA of such an objective lens is determined in accordance with a data density of a recording medium. For example, the NA of an objective lens of an optical pick-up for a CD (compact disc) is approximately 0.45. The NA of the objective lens for a DVD (digital versatile disc) is approximately 0.6.
The objective lens of the CD drives or DVD drives is generally a single lens formed by plastic molding, and having aspherical surfaces as both refraction surfaces. The objective lens for the CD or DVD drive is required such that spherical aberration is well compensated for in order to converge an incident light beam as a diffraction limited spot.
Further, coma should also be compensated counting decentering status of the objective lens due to manufacturing and/or assembly errors. To meet the above requirements, the conventional objective lens, which is typically a single lens having aspherical surfaces, is designed such that the spherical aberration is compensated in a predetermined reference status (which is generally a status where parallel light is incident on the objective lens), and sine condition is satisfied.
Recently, an optical disc having data recording density higher than that of the DVD is suggested. For such an optical disc, the NA of the objective lens is required to be 0.7 or more. However, if a focal length of the lens is shortened in order to raise the NA, if an objective lens is a single lens element, the curvature of the lens is higher, which is difficult to form accurately according to a current processing technique.
Japanese Patent Provisional Publication No. HEI 11-190818 discloses a high NA objective lens in which curvature of each lens surface is suppressed by constituting the objective lens with two lens elements.
However, such an objective lens consisting of two lens elements is larger in weight and volume in comparison with the objective lens having a single lens element. Therefore, for such a lens having two lens elements, a conventional fine actuator which moves the objective lens in its axial direction for focusing can not be used.
Further, the two lens elements must be fixed onto a frame and an optical axes of the lens elements must be aligned with respect to each other. In such a case, the number of manufacturing processes and the number of components may increase. Further, a working distance (i.e., a distance between a rear surface of the objective lens and a surface of a cover layer of an optical disc) of the objective lens disclosed in the publication is a range of 3.5 xcexcm through 50 xcexcm. This working distance is significantly smaller than that of a single-element lens having the same focal length.
An error of a thickness of a cover layer of an optical disc varies depending on manufacturing methods. It is difficult to reduce the error of the thickness of the cover layer less than 10 xcexcm according to the current technique. In the DVD standard, a tolerance of the thickness of the cover layer is xc2x10.03 mm.
If an optical disc includes an error in the thickness of the cover layer, spherical aberration is caused. The amount of the spherical aberration increases as the NA of the objective lens increases.
As described above, the conventional objective lens for CD""s or DVD""s has relatively low NA, and therefore, the amount of spherical aberration caused by the error of the thickness of the optical disc is relatively small. Accordingly, in the conventional optical pick-up, it is unnecessary to compensate for the spherical aberration caused by the error of the thickness.
However, for lenses whose NA is 0.7 or more, the amount of the spherical aberration caused by the thickness error of 10 xcexcm becomes impracticably large, and a diameter of a beam spot formed by the objective lens cannot be reduced to a practicable value if the spherical aberration is not compensated for.
In Japanese Patent Provisional Publication No. 2000-131603, technique for compensating for such a spherical aberration is disclosed. Specifically, according to the publication, the objective lens constituted of two lens elements disclosed in the afore-mentioned Publication No. HEI 11-190818 is used, and further, a compensation lens group is provided between the objective lens and a light source. The compensation lens consists of positive and negative lens elements. By adjusting a distance between the positive and negative lens elements of the compensation lens group, degree of divergence/convergence of light incident on the objective lens is adjusted so that the spherical aberration caused by the thickness error of an optical disc is compensated for.
Generally, when an objective lens, which is configured such that spherical aberration with respect to light having predetermined degree of divergence/convergence is compensated for, is used, the spherical aberration changes when the degree of divergence/convergence of the incident light changes. Therefore, spherical aberration caused by the thickness error of an optical disc can be canceled by changing the degree of divergence/convergence of the light incident on the objective lens to generate spherical aberration in an opposite direction.
It should be noted that in an optical system disclosed in the Patent Provisional Publication 2000-131603, spherical aberration caused by varying the degree of divergence/convergence of the incident light mainly consists of third order components, while the spherical aberration caused by the thickness error of an optical disc includes components higher than the third order. Therefore, even though the degree of divergence/convergence of the incident light is changed, the spherical aberration cannot be compensated for completely. In the publication, therefore, by varying a distance between the two lens elements of the objective lens, the higher order components of the spherical aberration is compensated for.
However, in order to adjust the distance between the lens elements of the objective lens, an adjusting mechanism should be provided on a lens frame of the objective lens, which is mounted on the fine actuator. Such a configuration requires a further burden to the actuator and troublesome adjusting operation.
The present invention is advantageous in that it provides an objective lens for an optical pick-up which enables sufficient correction of a spherical aberration caused by a thickness error of an optical disc only by changing the degree of divergence/convergence of the light incident on the objective lens.
According to embodiments of the, invention, there is provided a single-element objective lens for an optical pick-up. The objective lens directs an incident light beam to a data recording surface of an optical disc through a cover layer to form a beam spot thereon. A numerical aperture of the objective lens is 0.7 or more. The objective lens is configured to compensate for coma such that a characteristic of a change of spherical aberration due to a degree of divergence/convergence of the incident beam is substantially comparable with respect to a characteristic of a change of spherical aberration due to variation of the cover layer so that the change of spherical aberration due to variation of the cover layer can be cancelled by the change of spherical aberration due to a degree of divergence/convergence of the incident beam.
Optionally, an amount of an offence against a sine condition has a positive maximum value at a position within a range of 60% to 90% of an effective radius of the objective lens, the difference with respect to the sine condition monotonously decreasing on a peripheral side with respect to the position at which the difference has the maximum value.
Further optionally, the objective lens may be configured to satisfy condition:
0.001 less than SCmax/f less than 0.013,
where SCmax represents the positive maximum value of an amount of an offence against the sine condition when the incident beam is a parallel light beam, and f represents a focal length of the objective lens.
Alternatively or optionally, the objective lens is configured to satisfy condition:
xe2x88x920.37 less than xcex94W(1.0)xc3x97xcex/(fxc3x97mxc3x97NAmax6) less than xe2x88x920.25,
where, xcex94W(1.0) represents wavefront aberration in an outermost position of the effective aperture, xcex represents a wavelength of the light beam, m represents a lateral magnification, NAmax represents a numerical aperture corresponding to the effective aperture, and f represents a focal length of the objective-lens.
Alternatively or optionally, the objective lens is configured to satisfy condition:
0.050 less than f/r2 less than 0.110,
where, r2 represents a radius of curvature of a surface of the objective lens facing the optical disc, and f represents a focal length of the objective lens.
According to some embodiments, there is provided a single-element objective lens for an optical pick-up, the objective lens directing an incident light beam to a data recording surface of an optical disc through a cover layer to form a beam spot. A numerical aperture of the objective lens being 0.7 or more, and the objective lens is configured to compensate for coma such that spherical aberration caused by a change of a degree of divergence/convergence of the incident light beam is substantially proportional to a value OP expressed by equation:
OP=(1xe2x88x92{square root over ((1xe2x88x92NA2))})/nxe2x88x92(nxe2x88x92({square root over (n2xe2x88x92NA2))}),
where, NA represents a numerical aperture of the objective lens, and n represents a refractive index of the cover layer.
According to the embodiments, there is also provided an optical pick-up, which employs a laser source that emits a laser beam, any one of the single-element objective lenses described above, and an optical device capable of changing the degree of divergence/convergence of the laser beam incident on the objective lens.
Optionally, the laser source may include a laser diode that emits a divergent laser beam, and a collimating lens that collimates the divergent laser beam emitted by the laser diode, the collimated laser beam being emitted from the laser source to the objective lens. The optical device includes the collimating lens, and the degree of divergence/convergence of the laser beam emitted by the laser source is changed by changing a distance between the laser diode and the collimating lens.